I'm unpersuaded by this study. I may have missed a part, but he doesn't seem to pay much attention to correlation between the MCAT and USMLE/COMLEX, which is not insignificant. Since people with lower board scores are more likely to go into FP, it's reasonable to expect that those with lower MCAT scores are, by extension, likely to do the same.
He also mentions an increase of a point in the MCAT biological sciences which he suggests is indicative of gaming the system. Again, I may have missed something, but the MCAT is curved; an increase in matriculant scores just indicates an increased selectivity, not necessarily changes in the applicant pool.
While there may very well be a relationship between students who underperform on the MCAT relative to other matriculants and the choice of FP, he doesn't make a persuasive argument that there's a cause and effect relationship. It would be far more interesting to see what specialty students would choose *before* taking the MCAT and correlate that with actual scores. Below-average test performance tends to self-select for primary care for all the obvious reasons.